Refactor live thread state and share pending signal derivation

[Emanuele-web04]

Summary

• move pending approval and user-input signal derivation into shared helpers used by both server snapshots and web live state
• update the web store to rely on the main live orchestration event path for turn diffs, session changes, interrupts, and reverts
• preserve and surface richer live thread state in the UI, including proposed-plan linkage and server-derived sidebar summary fields
• add coverage for shared thread signals and live thread/store behavior

Testing

• bun x vitest run apps/web/src/store.test.ts
• bun x vitest run apps/web/src/session-logic.test.ts
• bun x vitest run packages/shared/src/threadSignals.test.ts
• bun run fmt
• bun run lint
• bun run typecheck